There is well known a process of coating a support with a solution of a photopolymerizable composition composed of a compound having at least one ethylenically unsaturated double bond (hereinafter, the compound is referred to as an ethylenic compound), and a binder having a film-forming faculty, a thermal polymerization preventing agent, a piasticizer, etc., if necessary, to form a photosensitive material having a photopolymerizable composition layer (a photosensitive layer); image-exposing the photosensitive layer based on a desired original to cause a polymerization in the exposed portions and harden the portions; and then treating the photosensitive layer with an organic solvent or an alkali capable of dissolving the unhardened portions only of the photosensitive layer, whereby the unhardened portions are dissolved off to form an image composed of the hardened portions. There is also a well known process of placing under pressure the photosensitive material as described above on other image support (either the support of the photosensitive material or the image support is transparent) to form a laminated material; image-exposing the photosensitive layer from the side of the transparent support to cause a polymerization at the exposed portions of the photosensitive layer, whereby causing a change between the adhesion of the exposed portions to the support of the photosensitive material and the adhesion thereof to the image support, said change in adhesion being different from the relation of adhesion at the unexposed portions; and separating both supports from each other in the state that the unexposed portions of the photosensitive layer are stuck on one support and the exposed portions are on another support to form an image of the photosensitive layer on each support.
In these processes, benzil, benzoin, anthraquinone, or Michler's ketone is used as a photopolymerization initiator. However, the photopolymerization composition using such a photopolymerization initiator is responsive to ultraviolet rays having relatively short wave lengths but has a problem that the photopolymerizable composition is scarcely responsive to ultraviolet rays of long wave lengths and visible rays such as argon laser and does not show a polymerization initiating faculty by such rays. In other words, it is very desirable for image-forming technique to use visible rays as the image exposure light source in the foregoing image formation or laser light as a light source capable of performing a scanning exposure.
As examples of a compound providing a photopolymerizable composition responsive to argon laser light by the combination with a 1,3,5-triazine compound shown by the general formula ##STR3## wherein R.sup.3, R.sup.4, and R.sup.5 each represents a hydrogen atom, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, or an aralkyl group; at least one of them being a mono-, di-, or tri-halogen-substituted methyl group,
said triazine being used in the present invention, there are 5-[(arenoxazol-2-iridene)ethylidene]-2-thiohydantoin described in Japanese Patent Application (OPI) No. 95,687/79 and 5-[(arenothiazol-2-iridene)ethylidene]-4-oxo-2-thioxazoline described in Japanese Patent Application (OPI) No. 151,024/79.
These compounds may be particularly used as a photopolymerization initiator for photosensitive materials using argon laser as a light source but it is very desirable to discover a photopolymerization initiator having a higher light sensitivity than those of the foregoing compounds so that a laser having a lower output can be used.